The present invention relates to an electroless silver plating solution, and more particularly, to a silver plating solution which uses a novel reducing agent system which is low in toxicity and very stable.
Many types of reducing agents have previously been suggested. These have included sodium hypophosphite, hydrazine, reducing sugars, glyoxal, thiourea, sodium borohydride, formaldehyde, sodium thiosulfate, monomethylamine borane, dimethylamine borane, and others. None of these baths are truly stable or commercially useful on a large scale. Most have very short effective plating lives, often in the range of hours or a few days. The solution, once made, will spontaneously and rapidly plate all surfaces with which it is in contact, including the container (Pearlstein and Weightman, Plating, 61, 154-7).
Most of the presently known commercial electroless silver plating solutions contain ammonia either as a stabilizer, a main complexing agent, or both. This is known to result in a major problem, as silver-amine complexes are known to be very shock sensitive explosives when dried. Explosions have occurred even when a glass stir rod is lifted from against the side of a beaker, disturbing the dried film.
Electroless silver plating solutions are generally considered to be borderline catalytic electroless metals (Cheng, et al, Plating and Surface Finishing, 77,130-132 (1990). True electroless metals such as copper and nickel can continuously build total metal thickness to indefinitely thick coatings of 25 microns (0.001 inch) or more. The freshly deposited metal is fully catalytic and remains capable of initiating further electroless metal deposition. Electroless silver baths, by contrast, rapidly lose autocatalytic activity. The freshly deposited silver metal is rarely able to continue catalytic activity beyond 0.25 microns (0.000010 inch).